A variety of tools and other equipment are used in downhole, wellbore environments. For example, a progressing cavity pump may be utilized in producing petroleum and other useful fluids from production wells. When a progressing cavity pump system is used, production tubing is disposed within a wellbore to extend through the wellbore to the progressing cavity pump system disposed at a specific location within the well. The progressing cavity pump can be deployed or retrieved through the center of the production tubing, via a wireline or coiled tubing.
In operation, fluids contained in an underground formation enter the wellbore via perforations formed through a wellbore casing adjacent to a production formation. Fluids, such as petroleum, flow from the formation and collect in the wellbore. A progressing cavity pump moves the production fluids upwardly through the production tubing to a desired collection point.
A progressing cavity pump, consists of a single helical rotor which rotates inside a double internal helical stator. The rotor is typically made from a high strength steel while the stator is molded of an elastomeric material. When the rotor is placed within the stator, two chains of spiral cavities are formed. As the rotor turns, the cavities spiral up the length of the pump. Fluid within the cavities is carried along as the cavities progress up the length of the pump. Hence the name, progressing cavity pump.
A progressing cavity pumping system, typically includes a motor drivingly coupled to a progressing cavity pump. For oil field applications, the motor may be located on the surface and drivingly coupled from the surface down to a submergible progressing cavity pump in the wellbore. This is an example of a top-driven pumping system. Alternatively, the motor may be placed in the wellbore as part of an electrical submergible progressing cavity pumping system. Electric power is provided to a submergible electric motor drivingly coupled to a progressing cavity pump. The fluid displaced by the pump is communicated to the surface through production tubing. Spatial considerations among the pump, production tubing and motor encourage placement of the submergible electric motor below the progressing cavity pump. Such a system is an example of a bottom-driven pumping system.
A significant advantage of the progressing cavity pump is that the presence of gas in the fluid will not cause the progressing cavity pump to cavitate, as in other types of pumps. However, free gas in the fluid stream can occupy space in the cavities that could otherwise have been filled by desired liquids, such as oil. This reduces the pumps useful capacity and causes apparent pump inefficiency.
Rotary gas separators have been used to reduce the concentrations of gas in the fluid stream of submergible pumping systems utilizing other types of pumps, such as centrifugal pumps. Rotary gas separators use centrifugal force and differences in the specific gravities of fluids to separate a fluid into its constituent gases and liquids. Typically, the drive train of a submergible electric pumping system is coupled to the rotary gas separator. However, the drive train of a progressing cavity pump tends to produce oscillations and gyrations that propagate through the drive train during operation. Those oscillations and gyrations increase the stress on bearings supporting the drive train within the rotary gas separator and lead to a higher likelihood of bearing failure.
Additionally, the orientation of the motor, pump, and fluid intake in a bottom-driven system increases the complexity of using a rotary gas separator. Typically, in a bottom-driven system the system is oriented with the motor at the bottom of a tool string. The motor is coupled to the progressing cavity pump through a drive train. Fluid enters the system through a separate fluid intake that is located between the motor and the progressing cavity pump. Thus, the drive train coupling the motor to the progressing cavity pump must pass through the fluid intake to the progressing cavity pump. Consequently, the fluid intake and any other element between the motor and pump must provide structural support to the motor in order for the motor to provide torque to the pump. The structural member and torque requirements in a bottom-driven system, along with the oscillations and gyrations produced in a progressing cavity pumping system, must be factored into the design of any system incorporating a rotary gas separator into the tool string between the motor and pump.
Therefore, it would be advantageous to have a system that could reduce the quantity of gas pumped by a submergible electric progressing cavity pumping system without the use of a rotary gas separator.